Re: [Phys-l] another crackpot idea from the California legislative assembly

[John Denker <jsd@av8n.com>]

On 02/27/2011 02:18 PM, David Schuster wrote:
> As Boris Korsunsky said on physlrnr, this idea does not sound
> 'obviously ridiculous' to him, nor to Charles I think, nor to me. I'm
> not sure the roadway piezo-electric proposal can be dismissed simply
> by appeal to 'conservation of energy', without considering the
> various energy transformations involved, both with and without the
> roadway piezo-electric transducers in place.  Let's assume that some
> energy is normally dissipated as 'heat' in the roadway anyway as cars
> pass over it and deform it slightly. If some of this were to generate
> energy in piezo-electric transducers instead, then energy remains
> conserved, just in a more usable form now, without requiring more
> energy from the car. The roadway would not get as warm presumably? Of
> course I know nothing of the details or mechanism of the proposed
> system so correct me if I am missing something, but I'm just
> conserving energy without taking more from the car.

Well, it is ridiculous, whether or not you consider it "obviously 
ridiculous".  Obviousness is in the eye of beholder.

> If some of this were to generate
> energy in piezo-electric transducers instead, then ....

Is there the slightest reason to think the energy would go into 
the transducer "instead" of the road?  Why is not the road strain 
energy present _in addition to_ the transducer strain energy?

To say it another way:  If the road flexes under the tire, why 
will it not flex under the tire + transducer?

If you're going to stiffen the roadway before adding the transducers, 
why not just stiffen the roadway _without_ adding the transducers, 
and go to the bank with the resulting savings in mileage?   If you 
don't consider this option, you are making a completely invalid 
apples-and-oranges comparison.

==========

We could also pretend to be physicists and calculate the amount
of deflection of the roadway (in microns), calculate the energy
in the resulting strain field (in millijoules), and calculate
the resulting upper bound on the power (in millihorsepower).
Actually this is a grossly loose upper bound, because I reckon
90% or more of the energy in the strain field is recovered 
elastically.

If you write small, you can do that calculation on the back of
an envelope.  You can't quite do it using dimensional analysis 
alone, but a rough scaling argument is quite sufficient.
  http://www.av8n.com/physics/scaling.htm

Or you can just make the common-sense argument that the energy
in the roadway strain surely must be orders of magnitude smaller
than the energy in the /tire/ strain.  Rubber versus concrete is
really not much of a contest.  And the rolling resistance of a
properly inflated tire is a small part of the overall energy
budget of the car.

So the idea that there is an appreciable amount of energy available
and that we can prevent it from going into the roadway is at least 
two leaps removed from plausibility.  The next step is to consider
the capital cost and the fragility of the transducer network.  Do
we really need to go there?

=======================

I've got an idea:  Let California pass AB 306.  Let them allocate
$400,000.00 to the task.  Caltrans can give me the money, and I
will be happy to analyze the proposal and write the required report.